Specific phenotypes may drive an increased incidence of pediatric inflammatory bowel disease in South-Eastern Norway.

Objectives: Pediatric inflammatory bowel disease (PIBD) incidence has increased in recent decades but may be stabilizing, prompting exploration of incidence changes, disease distribution, and severity.

Methods: From 2017 to 2019, patients under 18 years with PIBD symptoms were recruited from nine hospitals in South-Eastern Norway for Inflammatory Bowel Disease in South-Eastern Norway III (IBSEN III), a population-based inception cohort study. The primary outcome was a diagnosis of any PIBD subtype as defined by revised Porto criteria. Paris classification system defined disease phenotypes, and descriptions of covariates were gathered from patients in IBSEN III.

Results: We identified 324 PIBD patients, with 216 consenting to the IBSEN III study. The crude incidence rate was 17.8 per 100,000 person-years (PY) (95% confidence interval [CI]: 15.9-19.8). Crohn's disease (CD) was found in 118 patients (54.6%); 48% had ileocolonic distribution, 40% had upper gastrointestinal disease, and 12.8% had perianal disease. Complications (stricturing and/or penetrating) were noted in 18%. Ulcerative colitis (UC) was diagnosed in 78 patients (36.1%), predominantly pancolitis (41%), with 30% having proctitis. One in five suffered severe disease. IBD unclassified was found in 20 patients (9.3%). PIBD incidence in those under 16 was 13.6/100,000 PY, up from 4.7 in the IBSEN study 27 years ago. Terminal ileitis (11%-23%) and proctitis (14%-25%) rose from IBSEN (1990-1993) to IBSEN III, while stricturing/penetrating disease changed insignificantly (17%-16%).

Conclusions: The incidence of PIBD has risen in South-Eastern Norway, with increased cases of terminal ileitis in CD, unchanged stricturing/penetrating disease, and increased proctitis in UC compared to the original IBSEN study.

Trial registration: ClinicalTrials.gov identifier: NCT02727959.